Room temperature energy-efficient spin-orbit torque switching in two-dimensional van der Waals Fe3GeTe2 induced by topological insulators

Two-dimensional (2D) ferromagnetic materials with unique magnetic properties have great potential for next-generation spintronic devices with high flexibility, easy controllability, and high heretointegrability. However, realizing magnetic switching with low power consumption at room temperature is challenging. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in an all-van der Waals (vdW) heterostructure using an optimized epitaxial growth approach. The topological insulator Bi2Te3 not only raises the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing the FGT to switch at a low current density of ~2.2×106 A/cm2. The SOT efficiency is ~2.69, measured at room temperature. The temperature and thickness-dependent SOT efficiency prove that the larger SOT in our system mainly originates from the nontrivial topological origin of the heterostructure. Our experiments enable an all-vdW SOT structure and provides a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future.


Structural characteristics and magnetic property of FGT, Bi2Te3, and Bi2Te3/FGT heterostructures
To understand the microstructure and surface morphology, the structural characteristics of FGT, Bi2Te3, and Bi2Te3/FGT heterostructure were measured, and the results are shown in Fig. S1. The atomic force microscopy (AFM) results corresponding to FGT and Bi2Te3 are shown in Fig. S1a and respectively. Therefore, we confirm that the high-quality crystal was obtained with no in-plane domains. Meanwhile, anomalous Hall measurement was carried out to characterize their magnetic properties, and typical results from FGT and Bi2Te3/FGT are shown in Fig. S1g-i, which clearly demonstrate the PMA features.

SOT-induced magnetic switching in Bi2Te3/FGT(3) heterostructure at 210 K
In addition to the current-induced magnetic switching in the Bi2Te3/FGT(3) heterostructure at 200 K in the main text, we carried out the identical SOT measurement at another temperature of 210 K.
Similarly, when applying a constant external in-plane field with sweeping the applied Jwrite, the SOT from the charge-spin conversion in Bi2Te3 can induce the magnetization switching of the FGT layer. 4 As the external magnetic field increases, the switching current density decreases. When the applied magnetic field is reversed, the SOT switching shows an opposite chirality. All these results well demonstrate the magnetization switching behaviors in the existence of SOT.

SOT-induced magnetic switching in Bi2Te3/FGT(3) heterostructures at 190 K
Utilizing the identical SOT measurement, we performed the current-driven magnetic switching on Bi2Te3(8)/FGT(3) at 190 K, and typical results are shown in Fig. S3. The switching chirality is the same as other temperatures when reversing the external in-plane magnetic field. Based on the above switching behavior, we summarize the temperature-dependent phase diagram of magnetic states in the presence of external magnetic field and current density, as already shown in the main text.

Temperature-dependent resistivity from Bi2Te3 and FGT thin films
To elucidate the resistivity properties of Bi2Te3 and FGT, we performed transport measurements by the physical property measurement system. The temperature-dependent resistivity from Bi2Te3 and FGT is shown in Fig. S4. It shows that the resistivity value of the 5nm-FGT increases with the decrease of temperature, which is due to the surface disorder caused by the thin film sample exposed to the air 3,4 .
The R-T curve shows more insulating behaviors, which is in sharp contrast with the metal characteristic of thick FGT in Fig. S4b. It further proves that the thickness dependence will affect the interface of the heterostructure. Fig. S4c displays the typical metallic behavior of 8nm-Bi2Te3 from 100 K to 300 K.

Magnetotransport properties of Bi2Te3/FGT heterostructure
To prove the high interfacial thermal conductance in the Bi2Te3 (8)

Harmonic Hall voltage measurement and SOT efficiency calculation in Bi2Te3(8)/FGT(4)
To accurately describe the damping-like SOT efficiency through a harmonic Hall measurement, two methods are usually employed for fitting the data, including large-field power-law fitting and small field derivation fitting 5,6 . Formula (1) is the extended Landau-Lifshitz-Gilbert (LLG) equation that includes two spin torque terms. Here, the ̂ is a unit vector of the magnetization direction, ̂ is the average spin direction of the electrons diffusing into the magnetic layer, is the Gilbert damping constant, is the gyromagnetic ration, − ⃗⃗ is the current independent effective magnetic fields, is the damping-like (Slonczweski-Berger) term and is the field-like term 5 However, in absence of the thermal-related effect, the latter case that followed formula (3) should be more accurate for estimating SOT efficiency, since the former case still included field-like effective term. The formula (3) can make it clear that and represent and , respectively. The prerequisite condition required by this equation is that the direction of equilibrium magnetization 8 doesn't deviate significantly from the z-axis 5 . Therefore, we employed the small field derivation fitting method for the Bi2Te3(8)/FGT(4) heterostructure to evaluate the damping-like SOT efficiency, since this sample shows negligible thermal-related effect and strong PMA feature at low temperature.
Compared to formula (3), the presence of field-like term in formula (2)

Estimation of Fermi level in Bi2Te3(8) structure at different temperature
To quantitatively analyze the position of the EF level and its influence on the SOT switching, the following equation was employed 11 : where ℏ is the reduced Plank constant, * ≈ 0.15 0 is the effective mass and is the bulk carrier density. Hence, we further conduct the Hall measurements to estimate the bulk carrier density at different temperatures as shown in Fig. S7a. The illustration of the inset elucidates the band structure of Bi2Te3 and the location of the EF, which more explicitly reveals the source of Bi2Te3 carriers 12 . The EF is located in the bulk conduction band, above the Dirac-cone of surface state 13 . The fact that TI with reduced bulk conductance leads to a higher SOT efficiency suggests that the TSS renders significant contributions to the efficient SOT. Figure. S7b displays the temperature-dependent EF values for estimating its position related to the bulk conduction band and the inset presents the simplified diagram. To comprehensively analyze the physical mechanism of realizing SOT switching, we further prepared the Bi2Te3(6)/FGT(4) and Bi2Te3(10)/FGT(4) heterostructures and carried out the harmonic measurements. Figure S8 displays the in-plane magnetic field-dependent first and second harmonic Hall resistance signals from these two samples, meanwhile the signal from Bi2Te3 (8)